There is conventionally known an ink-jet type printer that scans a print head having a nozzle array for ejecting ink droplets in the main scanning direction perpendicular to the nozzle array and, at the same time, ejects ink droplets from the nozzle array onto a paper sheet, thereby printing an image onto the paper sheet.
A printer of such a type can print an image corresponding to a band-like print area (band) that is a unit area for printing and has the same width as the nozzle width (length of the nozzle array) in a single main scan of the print head. To print an image onto an area wider than a single band (e.g., a sheet of paper), band-by-band image print operation is repeated while the position of a paper sheet is shifted in the sub-scanning direction.
The number of gradations that can be expressed by ink droplets ejected from the print head is smaller than the number (e.g., 256 gradations) of gradations of an original image data corresponding to an image to be printed. Thus, ejection of the ink droplets by the print head is performed based on print data with a small number of gradations generated by halftone process.
There exists a problem with the image printed in such a printer that a high-density streak can be generated at the joint between adjacent two bands printed through different main scans. In order to cope with this problem, there has been proposed a technique in which pixels around the joint in the print data after halftone process are thinned or a technique in which the diameter rank of ink droplets corresponding to pixels around the joint area is reduced to reduce the ink amount. That is, in these techniques, the print data after halftone process is subjected to a reduction in the amount of ink to be used for printing the area around the joint between the two bands printed through different main scans so as to reduce the streak that can be generated around the joint.